Directional receiver coupling arrangement with frequency selectivity and gain control for DSL

ABSTRACT

A directional coupler for a modem has a high common mode noise rejection characteristic with low transmit signal leakage, and reduced distortion of components mixed with a received signal. A primary filter network is connected to a communications circuit. The primary filter network has an output connected to an input of an amplifier. A feedback network is connected between the input and output of the amplifier. The primary filter network, the amplifier and the feedback network are arranged to provide a virtual ground at the input of the amplifier, and a filter characteristic which attenuates frequencies in the transmit signal band. The advantage is reduced component count while providing excellent transmit signal rejection and reducing received signal distortion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationNo. 09/466,663 filed Dec. 20, 1999 now U.S. Pat. No. 6,681,012, whichclaims benefit under 35 U.S.C. 119(e) of U.S. Provisional PatentApplication No. 60/155,733 filed on Sep. 23, 1999.

MICROFICHE APPENDIX

Not applicable.

TECHNICAL FIELD

The invention relates in general to data signal processing and, inparticular, to a hybrid circuit within a modem for coupling to atelephone line to improve received signal discrimination.

BACKGROUND OF THE INVENTION

Most users of data communications services access data communicationsnetworks (e.g. the Internet) using dial-up connections establishedthrough the Public Switched Telephone Network (PSTN). The PSTN is stillsubstantially an analog communications network designed, before theadvent of digital communications, to transmit sounds in the audiblerange of the human voice.

Digital data is transported across the PSTN by converting the data intoan analog signal that is transmitted by varying, or modulating, thefrequency, phase, amplitude or other characteristic of a carrier signal.The modulation is performed by a modem attached to a standard telephoneline referred to as a “local loop”. When analog signals are receivedfrom other modems in the PSTN, the receiving modem performs an oppositefunction by demodulating the received analog signal to convert it backinto digital data.

Analog signals are simultaneously transmitted and received by the modemthrough the local loop. Thus the local loop carries a mixed signal thatincludes a combination of both transmit (Tx) signals being sent by themodem, and receive (Rx) signals being received by the modem. The modemmust therefore separate the Tx and Rx signals so that a substantiallyuncorrupted Rx signal can be supplied to the demodulation portion of themodem. The separation of the Tx and Rx signals is commonly performedusing a circuit known as a hybrid circuit, which is located between themodem's modulation/demodulation circuits and the local loop.

As is well known in the art, a classical hybrid circuit includes a linedriver that energizes a line transformer through an impedance (typicallya resistor) chosen to provide an appropriate termination impedance forthe local loop (which is usually a complex value). In order to separatethe Rx signals from the mixed signal on the local loop, the signal lineis tapped at a first tap point (proximal the line transformer) andsupplied to a summing circuit (typically a differential amplifier). TheTx signal is also supplied to the summing circuit by tapping at a secondtap point in a compensation network. The purpose of the compensationnetwork is to provide a branch line from the line driver (source of Txsignals) in which the strength of the Tx signal is directly proportional(preferably equal) to the Tx signal strength at the first tap point,while the Rx signal strength is strongly attenuated. The summing circuitcan then isolate the Rx signal from the mixed signal by finding adifference between the signals obtained at the first and second tappoints.

In practice, the effectiveness of this classical hybrid circuit dependson the match between the impedance of the line (Z_(LINE)) and that ofthe compensation network (Z_(BAL)). In particular, if Z_(BAL)=k*Z_(LINE)(where k is a constant) at all frequencies, then the summing circuitwill completely remove the Tx signal from the mixed signal, and none ofthe Tx signal will “leak” past the summing circuit.

However, the impedance of the local loop cannot be matched by a genericcompensation network, because it depends on several factors (e.g. looplength, physical properties, loop topology, and the presence of bridgedtaps, etc.) that are termination-specific. Consequently, in practice,Z_(BAL)<>k*Z_(LINE) and at least some of the Tx signal will leak pastthe summing circuit. This Tx signal leakage appears as noise in the Rxsignal supplied to the demodulator of the modem. This noise (Tx leakage)may include distortion components generated by the line driver, and mayalso cause distortions in the demodulator that fall in the Rx signalfrequency band and cannot be separated from the useful Rx signal.

Additionally, at a minimum usable Rx signal strength, common-mode noiseoriginating from multiple sources (e.g., Vcc, ground, capacitivecoupling, etc.) may rival the Rx signal strength. If this occurs, the Rxsignal integrity will be severely compromised, resulting in anunacceptably low signal-to-noise ratio.

It is known that Tx signal leakage can be reduced by inserting filtersbetween the tap points and the summing circuit. The filters can be tunedto attenuate signals in the Tx frequency band, without attenuating theRx signals. While such filters reduce Tx signal leakage, the filtercircuits may induce the same distortion problems discussed above,resulting in distortion components (within the Rx frequency band) thatare applied to the inputs of the summing circuit. As is well known,those distortion components cannot be separated from the useful Rxsignal. In addition, the filter circuits do not address problemsassociated with common-mode noise.

As taught in U.S. Pat. No. 5,822,426 (Rasmus et al.), which issued Oct.13, 1998, the problem of common-mode noise can be addressed by using abalanced pair of line drivers, respectively generating complementary Tx+and Tx− signals (or by inverting the mixed signal derived at the second(Tx) signal tap). This effectively eliminates common-mode noise suppliedto the input of the summing amplifier that extracts the Rx signal.However, Rasmus et al. do not address the problems of Tx signal leakageor distortion.

Accordingly, there remains a need for an improved circuit for coupling amodem receiver (demodulator) to a signal line that carries transmit andreceive signals simultaneously.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention is to provide adirectional coupler having high common-mode noise rejection and lowtransmit signal leakage, while substantially avoiding distortion ofcomponents mixed with a received signal.

Accordingly, an aspect of the present invention provides a directionalcoupler for coupling a signal receiver to a communications circuithaving a signal line adapted to simultaneously carry a transmit (Tx)signal sourced from a line driver circuit and a receive (Rx) signal anda compensation network connected to an output of the line driver forbalancing an impedance of the signal line. The directional couplercircuit comprises a primary network connected to the communicationscircuit and having a respective primary network output; an amplifierhaving a respective amplifier input connected to the primary networkoutput, and a respective amplifier output connected to the receiver; anda feedback network connected between the amplifier input and theamplifier output. The primary network, the amplifier, and feedbacknetwork are arranged to provide a virtual ground at the amplifier input.

A further aspect of the present invention provides a modem comprising: acommunications circuit for coupling a signal source of the modem to atelecommunications network, the communications circuit comprising asignal line adapted to simultaneously carry a transmit (Tx) signalsourced from a line driver circuit and a receive (Rx) signal having afrequency differing from that of the Tx signal and a compensationnetwork connected to an output of the line driver for balancing animpedance of the signal line; and a directional coupler for coupling asignal receiver of the modem to the communications circuit. Thedirectional coupler circuit comprises: a primary network connected tothe communications circuit and having a respective primary networkoutput; an amplifier having a respective amplifier input connected tothe primary network output, and a respective amplifier output connectedto the receiver; and a feedback network connected between the amplifierinput and the amplifier output. The primary network, the amplifier, andfeedback network are arranged to provide a virtual ground at theamplifier input.

Preferably, the primary network and feedback network are adapted tocooperate to provide a notch filter characteristic of the directionalcoupler circuit, the notch filter characteristic having a centerfrequency substantially corresponding to a frequency of the Tx signal.Alternatively, the primary network and feedback network can be adaptedto cooperate to provide a 2nd order chebychev filter characteristic.

In an embodiment of the invention, the primary network comprises: afirst input connected to the signal line for receiving a mixed signal; asecond input operatively connected to the compensation network forreceiving a compensation signal; and a filter network connected betweenthe first and second filter inputs and the primary network output.

In an embodiment of the invention, the filter network comprises apartially-split 4-port RC network connected between the first and secondinputs and the primary network output. Preferably, the partially-split4-port RC network comprises: an output portion comprising a resistor anda capacitor connected in parallel to the primary network output; and apair of input portions connected between respective ones of the firstand second inputs and the output portion, each input portion comprisinga resistor connected in series with the resistor of the output portion,and a capacitor connected in series with the capacitor of the outputportion. Still more preferably, the partially-split 4-port RC networkfurther comprises: a resistor connected between ground and a junctionbetween the capacitors of the input and output portions; and a capacitorconnected between ground and a junction between the resistors of theinput and output portions.

In another embodiment of the invention, the primary network comprises a4-port RC network connected to the first and second inputs viarespective first and second resistances. Preferably, the 4-port RCnetwork comprises first and second branches connected in parallel, thefirst branch comprising a pair of series connected resistors and acapacitor connected between ground and a junction between the seriesconnected resistors; and the second branch comprising a pair of seriesconnected capacitors and a resistor connected between ground and ajunction between the series connected capacitors.

In a still further embodiment of the invention, the primary networkcomprises: a first 4-port RC network connected between the first inputand the primary network output; and a second 4-port RC network connectedbetween the second input and the primary network output. Preferably,each 4-port RC network comprises first and second branches connected inparallel, the first branch comprising a pair of series connectedresistors and a capacitor connected between ground and a junctionbetween the series connected resistors; and the second branch comprisinga pair of series connected capacitors and a resistor connected betweenground and a junction between the series connected capacitors.

In an embodiment of the invention, the feedback network comprises firstand second branches connected in parallel, the first branch comprising apair of series connected resistors and a capacitor connected betweenground and a junction between the series connected resistors; and thesecond branch comprising a capacitor.

In an embodiment of the invention, the primary network output comprisesa controllable gain stage. Preferably, the controllable gain stagecomprises an attenuation network connected in parallel with a gaincontrol switch. The attenuation network is preferably adapted toattenuate a signal substantially without altering the filtercharacteristic of the directional coupler. The attenuation network cancomprise a pair of series connected resistors, and a capacitor connectedin parallel with one of the pair of series connected resistors.Preferably, the gain control switch is adapted to selectively bypass theattenuation network.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIGS. 1 a to 1 c schematically illustrate basic elements in adirectional coupler in accordance with an embodiment of the presentinvention;

FIG. 2 schematically illustrates a modem hybrid circuit incorporatingthe directional coupler shown in FIG. 1;

FIGS. 3 a and 3 b schematically illustrate a modem hybrid circuitincorporating a directional coupler in accordance with a secondembodiment of the present invention;

FIG. 4 schematically illustrates a fully balanced hybrid circuitincorporating a pair of directional couplers in accordance with theembodiment shown in FIG. 3 a;

FIG. 5 schematically illustrates a second fully balanced hybrid circuitincorporating a pair of directional couplers in accordance with theembodiment shown in FIG. 3 a;

FIG. 6 schematically illustrates a modem hybrid circuit incorporating adirectional coupler in accordance with a third embodiment of the presentinvention; and

FIG. 7 schematically illustrates a modem hybrid circuit incorporating adirectional coupler in accordance with a fourth embodiment of thepresent invention.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. However, itwill be obvious to those skilled in the art that the present inventionmay be practiced without such specific details. In other instances,well-known circuits have been shown in block diagram form in order notto obscure the present invention in unnecessary detail. For the mostpart, details concerning timing considerations and the like have beenomitted inasmuch as such details are not necessary to obtain a completeunderstanding of the present invention and are within the skills ofpersons of ordinary skill in the art.

The present invention provides a directional coupler circuit 2, as shownin FIG. 1 a, usable in a hybrid circuit of a modem. The directionalcoupler 2 enables the modem receiver (demodulator) to be connected to acommunication circuit 4 of a frequency-duplexed two wire transmissionsystem. The directional coupler 2 achieves both directional isolationand significant frequency discrimination to reduce the level oftransmitted signal contamination which must be handled by the receiver.The directional coupler 2 generally comprises a primary stage 6; anamplifier 8 coupled between the primary stage 6 and the modem receiver(not shown); and a feedback network 10 coupled between the output 12 andinput 14 of the amplifier 8. Preferably, the primary stage 6 includesfirst and second inputs 16, 18 for connection with the communicationscircuit 4; a primary stage output 20 connected between an amplifier 8and feedback network 10; and at least one filter network coupling thefirst and second inputs 16, 18 to the primary stage output 20. In theembodiment of FIG. 1 a, the primary stage comprises a pair of filternetworks 22 a, 22 b connected in parallel.

As shown in FIG. 1 b, each filter network 22 is a 4-portresistor-capacitor (RC) network consisting of a resistor branch 24connected in parallel with a capacitor branch 26. The resistor branch 24includes a pair of series connected resistors R1 and R2, and a capacitorC3 connected between ground and a junction 28 between the two resistorsR1 and R2. Similarly, the capacitor branch 26 includes a pair of seriesconnected capacitors C1 and C2 and a resistor R3 connected betweenground and a junction 30 between the two capacitors C1, C2. Thoseskilled in the art will understand that the filter network 22 has thesame structure as a twin-T notch filter. In fact, the filter network 22preferably provides a notch filter characteristic, and preferablyattenuates frequencies in the frequency band of transmit signals sent bythe modem through the communications circuit 4. However, the behavior ofthe filter network 22 differs significantly from that of a classicaltwin-T notch filter, in that the shape of the filter characteristic isdetermined by an interaction between the filter network 22 and thefeedback network 10 of the directional coupler 2. The presence of thevirtual ground 20 at the amplifier input 14 also has an impact on itsbehavior.

As shown in FIG. 1 c, the feedback network 10 preferably includes amodified 4-pole RC network having a resistor branch 32 and a capacitorbranch 34. As in the primary stage filter network 22 described above,the resistor branch 32 comprises a pair of series connected resistors R4and R5, and a capacitor C5 connected between ground and the junction 36between the two resistors R4, R5. However, in the feedback network 10,the capacitor branch 34 comprises a single capacitor C4 connected inparallel with the resistor branch 32, and lacks a ground connection.This structure of the feedback network 10 also provides a filteringcharacteristic, and preferably also attenuates signals in the frequencyband of the transmit signals. As mentioned above, the overall shape ofthe filter characteristic of the directional coupler 2 is determined byinteraction between the feedback network 10 and the primary stage filternetwork(s) 22. Preferably, the overall shape of the notch filtercharacteristic is a 2nd order elliptic function. However, a 2nd orderchebychev filter characteristic is also usable in the present invention.

The amplifier 8, which may be a conventional operational amplifier, isconnected between the primary stage output 20 and the modem receiver(demodulator—not shown), in parallel with the feedback network 10.

The overall structure of the directional coupler 2 illustrated in FIGS.1 a–c may be referred to as a “single feedback bi-quad” having thefollowing characteristics:

-   -   a virtual ground at the amplifier input 14 eliminates common        mode voltage at the amplifier input 14;    -   signals input at the first and second primary stage inputs 16,18        are summed, to thereby enable extraction of receive signals from        the mixed signals in the communications circuit 4; and    -   the primary stage filter network(s) 22 and the feedback network        10 cooperate to provide significant filtering (filtering power        of a complex pole plus a complex zero) and gain reduction        applied to the signal prior to the amplifier 8. This reduces the        voltage handling and linearity requirements of the amplifier 8        and the modem receiver (not shown), with consequent reductions        in power consumption.

It will be understood that there are many circuits which possess avirtual ground, and therefore reduce distortion possibly due to commonmode effects. However, the single feedback bi-quad structure of thepresent invention has a further advantage that current input to thesumming point from the signals at the primary stage inputs 16, 18 isminimized over a significant part of the overall reject band. Thisreduces the current which must be provided from the amplifier output 12via the feedback network 10, and thus further reduces the possibility ofdistortion. Additionally, the amplifier 8 does not need to exhibit ahigh open-loop gain in this part of the band.

FIG. 2 illustrates the directional coupler of FIGS. 1 a–c connected to aconventional communications circuit (hybrid) of a modem. In theillustrated embodiment, the communications circuit 4 includes a signalline 38 and a compensation network 40. The signal line 38 comprises atransmit line driver 42 which drives a line transformer through animpedance (e.g., a resistance) R₀. For convenience, the impedance of theline transformer and local loop are represented by an impedance (e.g.,principally an inductance) Z_(LINE). The compensation network 40 isconnected to the output of the line driver 42, and is designed tobalance R₀ and Z_(LINE). Thus in the illustrated embodiment, thecompensation network 40 consists of impedances kR₀ and Z_(BAL), whichare arranged symmetrically about the line driver 42 with respect to R₀and Z_(LINE).

As shown in FIG. 2, the directional coupler 2 is connected to thecommunications circuit 4 by connecting the first and second primarystage inputs 16, 18 to the communications circuit at points A and Brespectively. The first input 16, connected at point A, receives a mixedsignal carried on the signal line 38, including the transmit (Tx) signalgenerated by the line driver 42, and receive signals received throughthe line transformer. The second input 18 is coupled to the compensationnetwork 40 at point B, and thus receives a compensation signal. Byvirtue of the symmetry between the signal line 38 and compensationnetwork 40, the compensation signal contains the Tx signal having astrength directly proportional (preferably equal) to the transmit signalcomponent of the mixed signal obtained at point A of the signal line 38.However, receive signals received through the line transformer areattenuated by R₀ and kR₀, so that the level of the receive signalcomponent of the compensation signal is significantly lower than that ofthe mixed signal. The compensation signal is preferably inverted by aninverter 44 which, if desired, may also act to adjust the amplitude ofthe compensation signal prior to its being applied to the primary stagefilter network 22 b.

Using the above structure, the receive signals can be extracted from themixed signal by summing the mixed and compensation signals received atthe first and second inputs 16, 18 of the primary stage 6. Thisarrangement eliminates common mode noise at the input 14 of theamplifier 8, so the receive signal (and the receiver itself) is wellprotected against common mode noise. Transmit signal leakage (due tounavoidable imbalance between the line impedance (Z_(LINE)) and thecompensation network impedance (Z_(BAL)) is effectively filtered by thecombination of the primary stage filter network 22 and feedback network10, before being applied to the amplifier input 14, so distortioncomponents are significantly reduced. The structure of the inventionpermits up to a second order elliptic function which has good rejectionproperties, further improving the distortion. The primary stage filternetwork(s) 22 and feedback network 10 of this single feedback bi-quadstructure can be configured to implement either a high pass or a lowpass response. Consequently, this same arrangement can be equallyapplied to a modem serving a host computer (e.g., at a customerpremises), and a modem in a central office, or some other remotelocation.

As shown in FIG. 3 the primary stage 6 can be readily modified toimplement receive signal gain control functionality, with either gain orattenuation. Preferably, this is accomplished by adding a controllablegain stage 46 connected between the filter network(s) 22 a, b and theinput 14 of the amplifier 8. The controllable gain stage 46 includes anattenuation network 34 (FIG. 3 b) connected in parallel with a gaincontrol switch 48. As shown in FIG. 3 b, the attenuation network 34preferably includes a pair of series connected resistors R6, R7, and acapacitor C6 connected in parallel with the resistor R6. Thisarrangement provides attenuation of the receive signal (of, for example,approximately 3 dB), without significantly altering the shape of thein-band filter characteristic provided by the combination of the filternetwork(s) 22 and feedback network 10. The gain control switch 48 isconnected to bypass the attenuation network 34, and thereby enablescontrol of receive signal attenuation by the modem.

FIGS. 4 and 5 show respective exemplary implementations in which thedirectional coupler 2 of FIG. 3 a is coupled for use in fully balancedcommunications circuits 4. As is known in the art, in a fully balancedcommunication circuit complementary transmit signals (Tx+ and Tx−) aregenerated by respective line drivers and used to drive opposite sides ofthe line transformer. A compensation network is provided for impedancematching, and is conveniently connected as a bridge between therespective signal lines carrying the complementary transmit signals.

In the embodiment shown in FIG. 4, each of the two signal lines 38 a, 38b (connecting respective line drivers 42 a, 42 b and the linetransformer (Z_(LINE)) includes a respective resistance R₀. Thecompensation network 40 forms a bridge between the signal lines 38 a, 38b, and, in order to maintain symmetry with the signal lines 38 a, 38 band line transformer Z_(LINE), includes an inductance Z_(BAL) seriesconnected between two resistances kR₀. Due to the symmetry of thecommunications circuit 4, and the fact that the transmit signal Tx− isinverted with respect to its complementary transmit signal Tx+, thesignals obtained at points A1 and B1 will be inverted with respect toeach other, and will also be inverted with respect to the correspondingsignals obtained at A2 and B2.

Using this arrangement, a receive signal Rx+ can be obtained byconnecting a directional coupler 2 a so that one of the primary stageinputs 16 is connected to a “positive” signal line 38 a at point A1 tothereby receive a “positive” mixed signal. The other primary stage input18 is connected to the compensation network 40 at point B1, andtherefore receives a “negative” compensation signal (i.e., which isinverted with respect to the “positive” mixed signal obtained at pointA1). Similarly, a “negative” receive signal Rx− can be generated byconnecting a second directional coupler 2 b to points A2 (“negative”mixed signal) and B2 (“positive” compensation signal) as shown in FIG.4. It will be apparent that the use of inverters 44 to invert thecompensation signals, as shown in FIGS. 2 and 3 a, are not necessarywith a fully balanced communication circuit 4, as inverted compensationsignals can be obtained by tapping the compensation network 40 atappropriate points.

The embodiment shown in FIG. 5 is similar to that shown in FIG. 4,except that each signal line 38 a, 38 b includes a respective linetransformer coil L1 and L2, which are separated by a resistance R₀. Inorder to maintain symmetry, the arrangement of inductances Z_(BAL) andresistances kR₀ of the compensation network 40 is reversed (with respectto the embodiment shown in FIG. 4). As in the embodiment of FIG. 4, acompensation signal obtained at point B1 of the compensation network 40is inverted with respect to the mixed signal obtained at point A1 ofsignal line 38 a. Similarly, a compensation signal obtained at B2 isinverted with respect to a mixed signal obtained at A2 of signal line 38b. Thus complementary receive signals Rx+ and Rx− can be extracted byconnecting respective directional couplers 2 a, 2 b to thecommunications circuit 4 in the manner discussed above with respect toFIG. 4.

FIGS. 6 and 7 respectively show alternative embodiments of a directionalcoupler 2 in accordance with the present invention. For simplicity, ineach case the directional coupler 2 is shown connected to a simple(unbalanced) communications circuit 4, in the manner illustrated inFIGS. 2 and 3. Furthermore, attenuation networks 46 have been omittedfrom these embodiments. It will be appreciated, however, that theembodiments of FIGS. 6 and 7 can be readily applied to fully balancedcommunications circuits 4, by connecting the primary stage inputs 16, 18to appropriate points on the signal line 38 and compensation network 40as discussed above with respect to FIGS. 4 and 5. Similarly, attenuationnetworks 46 can be readily inserted between the filter network 22 andthe amplifier input 14 if desired.

In the embodiment shown in FIG. 6, the primary stage is modified byeliminating one filter network 22, and connecting the remaining filternetwork 22 to both of the primary stage inputs 16, 18 via respectiveresistances R8, R9. This reduces the parts count and component cost, byremoving the problem of component matching between two parallel filternetworks 22 a, 22 b. However, the additional resistances, and the factthat the mixed and compensation signals are summed upstream of theremaining filter network 22, may result in some alteration of theoverall filter characteristics produced by the combination of theremaining filter network 22 and feedback network 10.

The embodiment shown in FIG. 7 enables a reduced parts count, andreduces the problem of component matching (both relative to the case oftwo parallel filter networks 22 a, b) by using a partially split filternetwork 22′. The filter network 22′ can be considered to have an inputend and an output end. Referring briefly to FIG. 1 a, the input endcomprises a resistor (R1) and a capacitor (C1) proximal the inputs 16,18 of the primary stage 6. The remainder of the filter network 22′,which consists of resistors R2 and R3, and capacitors C2 and C3, formsthe output end. As shown in FIG. 7, the input end of the filter network22′ is split into two equal input branches 50 and 52 both of which areconnected to the output end, which remains unmodified. Thus, the inputresistor R1 of the filter network(s) 22 (see FIG. 1 b) is replaced bytwo parallel resistances R1′, R1″, each having a resistance value twicethat of R1. The input capacitor C1 is replaced by two parallelcapacitors C1′, C1″ each having a capacitance value one half that of C1.

Each input branch 50, 52 of the partially split filter network 22′ isconnected to a respective primary stage input 16, 18. Thus one inputbranch 50 receives the mixed signal from the signal line 38 of thecommunication circuit 4, while the other input branch 52 receives thecompensation signal from the compensation network 40. By suitablymatching component values between each input branch 50, 52 of thepartially split filter network 22′, the original filter characteristics(i.e., the filter characteristics obtained with two parallel filternetworks 22) can be preserved. However, because the output end of thefilter network 22′ does not have to be duplicated, the total parts countand the difficulties associated with component matching between twoparallel filter networks 22 a, 22 b are reduced.

The embodiments of the invention described above are intended to beexemplary only. The scope of the invention is therefore intended to belimited solely by the scope of the appended claims.

1. A modem comprising: a) a communications circuit for coupling a signalsource of the modem to a telecommunications network, the communicationscircuit comprising a signal line adapted to simultaneously carry atransmit (Tx) signal and a receive (Rx) signal and a compensationnetwork connected to the signal line for balancing an impedance of thesignal line; and b) a directional coupler for coupling a signal receiverof the modem to the communications circuit, the directional couplercircuit comprising: i) a primary network connected to the communicationscircuit, the primary network being tuned to attenuate frequencies in afrequency band of the Tx signal, and having a respective primary networkoutput; ii) an amplifier having a respective amplifier input connectedto the primary network output, and a respective amplifier outputconnected to the receiver; and iii) a feedback network tuned toattenuate frequencies in a frequency band of the Tx signal, andconnected between the amplifier input and the amplifier output; whereinthe primary network, the amplifier, and feedback network are arranged toprovide a virtual ground at the amplifier input.
 2. The modem as claimedin claim 1, wherein the signal line comprises first and second signalline impedances connected in series between a transmit line driver andthe local loop, the first input being connected to the signal linebetween the first and second signal line impedances.
 3. The modem asclaimed in claim 2, wherein the first signal line impedance is aresistor, and the second signal line impedance is an inductance.
 4. Themodem as claimed in claim 3, wherein the second signal line impedancecomprises a coil of a line transformer connecting the signal line to thelocal loop.
 5. The modem as claimed in claim 2, wherein the compensationnetwork comprises first and second compensation impedances connected inseries, the second input being connected to the compensation networkbetween the first and second compensation impedances.
 6. The modem asclaimed in claim 5, wherein the first compensation impedance is aresistor, and the second compensation impedance is an inductance.
 7. Themodem as claimed in claim 1, wherein the primary network and thefeedback network are adapted to cooperate to provide a notch filtercharacteristic of the directional coupler, the notch filtercharacteristic having a center frequency substantially corresponding toa frequency of the Tx signal.
 8. The modem as claimed in claim 1,wherein the primary network comprises: a) a first input connected to thesignal line for receiving a mixed signal; b) a second input operativelyconnected to the compensation network for receiving a compensationsignal; and c) a filter network connected between the first and secondinputs and the primary network output.
 9. The modem as claimed in claim8, wherein the filter network comprises a partially-split 4-portresistor-capacity (RC) network connected between the first and secondfilter inputs and the primary network output.
 10. The modem as claimedin claim 9, wherein the partially-split 4-port RC network comprises: a)an output portion comprising a resistor and a capacitor connected inparallel to the primary network output; and b) a pair of input portionsconnected between respective ones of the first and second inputs and theoutput portion, each input portion comprising a resistor connected inseries with the resistor of the output portion, and a capacitorconnected in series with the capacitor of the output portion.
 11. Themodem as claimed in claim 10, wherein the partially-split 4-port RCnetwork further comprises: a) a resistor connected between ground and ajunction between the capacitors of the input and output portions; and b)a capacitor connected between ground and a junction between theresistors of the input and output portions.
 12. The modem as claimed inclaim 9, wherein the filter network comprises a 4-port RC networkconnected to the first and second inputs via respective first and secondresistances.
 13. The modem as claimed in claim 12, wherein the 4-port RCnetwork comprises first and second branches connected in parallel, thefirst branch comprising a pair of series connected resistors and acapacitor connected between ground and a junction between the seriesconnected resistors; and the second branch comprising a pair of seriesconnected capacitors and a resistor connected between ground and ajunction between the series connected capacitors.
 14. The modem asclaimed in claim 9, wherein the filter network comprises: a) a first4-port RC network connected between the first filter input and theprimary network output; and b) a second 4-port RC network connectedbetween the second filter input and the primary network output;.
 15. Themodem as claimed in claim 14, wherein each 4-port RC network comprisesfirst and second branches connected in parallel, the first branchcomprising a pair of series connected resistors and a capacitorconnected between ground and a junction between the series connectedresistors; and the second branch comprising a pair of series connectedcapacitors and a resistor connected between ground and a junctionbetween the series connected capacitors.
 16. The modem as claimed inclaim 9, wherein the primary network output comprises a controllablegain stage.
 17. The modem as claimed in claim 16, wherein thecontrollable gain stage comprises an attenuation network connected inparallel with a gain control switch.
 18. The modem as claimed in claim17, wherein the attenuation network is adapted to attenuate a signalsubstantially without altering a filter characteristic of thedirectional coupler.
 19. The modem as claimed in claim 17, wherein theattenuation network comprises a pair of series connected resistors, anda capacitor connected in parallel with one of the pair of seriesconnected resistors.
 20. The modem as claimed in claim 17, wherein thegain control switch is adapted to selectively bypass the attenuationnetwork.
 21. The modem as claimed in claim 1, wherein the feedbacknetwork comprises first and second branches connected in parallel, thefirst branch comprising a pair of series connected resistors and acapacitor connected between ground and a junction between the seriesconnected resistors; and the second branch comprising a capacitor.